A composite polycrystalline structure graphene lithium manganese iron phosphate and its preparation method and application

Abstract

The invention discloses a preparation method of lithium manganese iron phosphate with a composite polycrystalline structure. The method comprises the following steps: adding a phosphorus source and a carbon-containing substance in an amount of 3% to 10% by mass of the ferrous sulfate to a ferrous sulfate solution, reacting the mixture in an oxygen-free environment with a pH value of 5 to 10, and filtering to obtain a wet ferrous phosphate material containing carbon compounds; the carbon-containing substances comprising at least two substances; adding an oxidant and an aerogel to a manganese sulfate solution for reaction to obtain a manganese dioxide nanomaterial; mixing the graphene-containing ferrous phosphate wet material and the manganese dioxide nanomaterial, and reacting the mixture with a lithium salt and an adhesion promoter in an oxygen-free environment to obtain a lithium manganese iron phosphate precursor; and sequentially subjecting the lithium manganese iron phosphate precursor to ball milling, drying, sintering, and cooling to obtain the lithium manganese iron phosphate with the composite polycrystalline structure. The prepared lithium manganese iron phosphate has good charge and discharge performance, compaction density, and processing performance, and is worthy of promotion in the battery field.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a composite polycrystalline graphene lithium manganese iron phosphate and a preparation method and application thereof. Background Art

[0002] In recent years, lithium-ion batteries have achieved remarkable success in the application of consumer electronics, energy storage power stations, and electric vehicles. As one of the key materials in lithium-ion battery systems, cathode materials have garnered widespread attention. Commonly used materials, such as lithium iron phosphate, have excellent electrochemical properties, a very stable charge and discharge platform, a stable structure during the charge and discharge process, and advantages such as being pollution-free, non-toxic, safe, usable in high-temperature environments, and widely available raw materials. Therefore, it is a raw material that the battery industry is currently vying to develop. However, due to problems such as poor low-temperature performance, low tap density, poor consistency, and low energy density, these have become key factors restricting the large-scale development of lithium iron phosphate.

[0003] Compared with lithium iron phosphate, lithium manganese iron phosphate has the advantages of both lithium iron phosphate and lithium manganese phosphate. At the same time, lithium manganese iron phosphate has a higher discharge platform. + / Li electrode potential is only 3.4V, while lithium manganese iron phosphate has a + The electrode potential of / Li is 4.1V, which makes its theoretical energy density 15-20% higher than that of lithium iron phosphate under the same conditions, and the cost of the synthesis technology route is similar to that of current lithium iron phosphate; however, the existing lithium manganese iron phosphate has problems such as poor conductivity and cycle life, as well as an unstable discharge platform. Summary of the Invention

[0004] In view of the problems of poor conductivity and cycle times and unstable discharge platform of existing lithium manganese iron phosphate, the present invention provides a composite polycrystalline structure graphene lithium manganese iron phosphate and its preparation method and application.

[0005] On the one hand, the present invention discloses a composite polycrystalline structure lithium manganese iron phosphate, which is a composite polycrystalline structure graphene lithium manganese iron phosphate formed by doping at least two different carbon-containing substances including aerogel with an iron source, a manganese source and a lithium source under high-temperature calcination.

[0006] Further defined, the chemical formula of the lithium manganese iron phosphate is: LiMn x Fe y PO4 / C, where: 0.2≤x≤0.9, x+y=1.

[0007] The present invention has the following beneficial effects: By doping carbon particles of varying sizes to increase tap density, the synergistic effect of these different carbon particle sizes and the aerogel facilitates the stabilization of lithium ion channels, thereby improving the discharge capacity of the lithium iron manganese phosphate and establishing a stable discharge platform. These excellent tap density and discharge capacity demonstrate excellent cycling performance and rate capability. Furthermore, the addition of a minimal adhesion promoter significantly reduces the shedding of the carbon layer during subsequent processing, thereby enhancing the material's conductivity and processing performance.

[0008] The present invention also discloses a method for preparing composite polycrystalline lithium manganese iron phosphate, the preparation method comprising:

[0009] A phosphorus source and a carbon-containing substance whose mass is 3% to 10% of the mass of the ferrous sulfate are added to a ferrous sulfate solution, reacted in an oxygen-free environment with a pH value of 5 to 10, and filtered to obtain a wet ferrous phosphate material containing carbon compounds; the carbon-containing substance comprises at least two;

[0010] adding an oxidant to a manganese sulfate solution to react with the aerogel to obtain a manganese dioxide nanomaterial;

[0011] The graphene-containing ferrous phosphate wet material and the manganese dioxide nanomaterial are mixed and reacted with a lithium salt and an adhesion promoter in an oxygen-free environment to obtain a lithium manganese iron phosphate precursor;

[0012] The lithium manganese iron phosphate precursor is subjected to ball milling, drying, sintering and cooling in sequence to obtain the composite polycrystalline lithium manganese iron phosphate.

[0013] It is further defined that, in the process of preparing the lithium iron manganese phosphate precursor, the molar ratio of lithium ions in the lithium salt to iron ions in the ferrous phosphate wet material is 1.0-1.1:0.2-0.8, preferably 1.0-1.1:0.5.

[0014] It is further defined that the concentration of the ferrous sulfate solution and the concentration of the manganese sulfate solution are both 1 to 5 mol / L, preferably 3 to 5 mol / L.

[0015] It is further defined that an antioxidant is also added in the step of preparing the graphene-containing ferrous phosphate wet material. Preferably, the antioxidant includes one or more of citric acid, ascorbic acid and phytic acid.

[0016] It is further defined that the carbon-containing substance includes at least one of graphene, glucose, phytic acid, citric acid, sucrose and tartaric acid.

[0017] It is further defined that the carbon-containing material is a mixture of two carbon-containing materials with different particle sizes; the mass ratio of the two carbon-containing materials with different particle sizes is 3-7:1-6, preferably 3-7:4-6.

[0018] It is further defined that, in the process of preparing the manganese dioxide nanomaterial, the amount of the aerogel added is 1% to 7% of the mass of manganese in the manganese sulfate solution.

[0019] It is further defined that the aerogel includes at least one of graphene aerogel, carbon aerogel, organic aerogel and V2O5 aerogel.

[0020] It is further defined that the adhesion promoter includes at least one of sodium fatty alcohol polyoxyethylene ether sulfate, dicyclohexylamine, ethylene glycol, sodium lauryl sulfate and dimethylacetamide.

[0021] It is further defined that in the step of preparing the graphene-containing ferrous phosphate wet material, the reaction temperature is 50-80°C, preferably 50-60°C; preferably, the amount of the phosphorus source added is 100%-110% of the theoretical added mass; preferably, in the process of preparing the manganese dioxide nanomaterial, the amount of the oxidant added is 100%-110% of the theoretical molar amount; preferably, the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid; preferably, the lithium salt includes at least one of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, lithium hydroxide and lithium chloride; preferably, the oxidant includes at least one of oxygen, air, ammonium persulfate, hypochlorite, chlorate and permanganate.

[0022] Beneficial effects of the present invention:

[0023] 1. The present invention facilitates the structural transformation of aerogels and other carbon compounds into nanopores and graphene network structures through sintering, forming a dense carbon coating layer with iron, manganese and lithium sources, thus providing the material with good energy storage effect;

[0024] 2. The synergistic effect of aerogel and carbon-containing materials helps stabilize the lithium ion channel, ensuring the normal insertion and extraction of lithium ions, significantly improving the cycle performance and discharge rate. At the same time, the lithium manganese iron phosphate prepared by the method disclosed in the present invention has controllable, consistent and good conductivity;

[0025] 3. The addition of a very small amount of adhesion promoter can greatly improve the powder loss phenomenon of the carbon layer during subsequent processing of the material, thereby increasing the conductivity and processing performance of the material.

[0026] 4. The lithium manganese iron phosphate prepared by the present invention has uniform particle size distribution, excellent battery energy density, compaction density and good charge and discharge performance, and is worthy of promotion in the battery field. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM electron microscope image of the lithium manganese iron phosphate prepared in Example 1. DETAILED DESCRIPTION

[0028] When the inventors of the present invention used existing lithium manganese iron phosphate to replace traditional lithium iron phosphate, they found that the existing lithium manganese iron phosphate had problems such as poor conductivity and cycle number and unstable discharge platform.

[0029] In the process of solving the above problems, the inventors accidentally discovered that doping carbon-containing substances of different particle sizes into the existing lithium manganese iron phosphate is beneficial to increasing the tap density. They also found that by adding aerogel during the preparation process of lithium manganese iron phosphate and sintering it, the obtained lithium manganese iron phosphate has stable lithium ion channels and good energy storage effects, showing controllability, consistency and good conductivity.

[0030] The present invention discloses a composite polycrystalline structure of lithium manganese iron phosphate, which is doped with at least two different carbon-containing materials including aerogel, an iron source, a manganese source, and a lithium source, and calcined at high temperature to form a composite polycrystalline structure of graphene lithium manganese iron phosphate. It can be represented by the following chemical formula: LiMn x Fe y PO4 / C, wherein: 0.2≤x≤0.9, x+y=1, and “ / C” indicates doping with carbon.

[0031] In the present invention, different carbon-containing materials have different structures after carbonization, and the obtained carbonized products are also different, especially the particle sizes of the carbonized products are greatly different. Therefore, using two different carbon-containing materials can obtain lithium manganese iron phosphate with a larger high-pressure density.

[0032] In the present invention, the “carbon-containing substance” may be at least two of graphene, glucose, phytic acid, citric acid, sucrose and tartaric acid.

[0033] In the present invention, the "aerogel" can be at least one of graphene aerogel, carbon aerogel, organic aerogel and V2O5 aerogel.

[0034] The present invention also discloses a method for preparing composite polycrystalline lithium manganese iron phosphate, comprising the following steps:

[0035] S1. A phosphorus source and a carbonaceous substance are added to the ferrous sulfate solution, reacted in an anaerobic environment at a pH of 5 to 10 (favorable for precipitation of ferrous phosphate), and filtered to obtain a wet ferrous phosphate containing carbon compounds;

[0036] As used herein, the "phosphorus source" refers to a substance that can provide phosphorus elements, such as phosphoric acid and phosphates; in the present invention, the added mass of the "phosphorus source" can be 100% to 110% of the theoretical value, suitably, 105% to 110%; in the present invention, the "phosphorus source" includes but is not limited to at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid.

[0037] As used herein, the "carbon-containing substance" refers to a compound that can provide carbon elements, such as graphene, glucose, phytic acid, citric acid, sucrose and tartaric acid; in the present invention, the carbon-containing substance consists of at least two, such as two, three, four, etc. When there are two, the mass ratio of the two carbon-containing substances can be 3-7:1-6, suitably, 3-7:4-6.

[0038] In the present invention, the concentration of the "ferrous sulfate solution" can be 1 to 5 mol / L, suitably 3 to 5 mol / L.

[0039] In the present invention, in order to prevent Fe 2+ Oxidized, so an antioxidant can be added. As used herein, the "antioxidant" refers to an agent that can react with oxygen to consume oxygen and avoid the reaction of oxygen with Fe 2+ reaction, such as citric acid and ascorbic acid, etc. In the present invention, the antioxidant can be at least one of citric acid, ascorbic acid and phytic acid; In the present invention, the added mass of the antioxidant can be 0.01% to 1% of the mass of the ferrous sulfate, suitably, 0.3% to 1%, more suitably, 0.7% to 1%.

[0040] In the present invention, the pH value of 5 to 10 can be adjusted by using concentrated ammonia water, the mass fraction of which is 5% to 25%. In this step, in order to increase the yield of ferrous phosphate and reduce Fe 2+ The reaction temperature may be 50-80°C, preferably 50-65°C, and most preferably 50-55°C.

[0041] In the present invention, the "filtration" may be a suction filtration method, specifically a vacuum pump filtration method.

[0042] S2. adding an oxidant and aerogel to a manganese sulfate solution to react to obtain a manganese dioxide nanomaterial;

[0043] In the present invention, the concentration of the "manganese sulfate solution" can be 1 to 5 mol / L, suitably 3 to 5 mol / L;

[0044] In the present invention, the "oxidant" may be at least one of oxygen, air, ammonium persulfate, hypochlorite, chlorate and permanganate, and the amount of the oxidant added may be 100% to 110% of the theoretical molar amount.

[0045] In the present invention, the solute in the "manganese sulfate salt solution" can be derived from at least one of potassium permanganate, potassium manganate, manganese sulfate, manganese chloride, manganese oxalate and manganese acetate, preferably potassium permanganate.

[0046] In the present invention, the "aerogel" can be at least one of graphene aerogel, carbon aerogel, organic aerogel and V2O5 aerogel, preferably graphene aerogel. The added mass of the aerogel can be 1% to 7% of the mass of the manganese element in the manganese sulfate salt solution; the aerogel undergoes structural changes at high temperature.

[0047] In the present invention, the manganese dioxide nanomaterial can be synthesized by a hydrothermal method by adding an oxidant to the manganese sulfate solution and reacting with the aerogel.

[0048] S3. The wet material of the carbon compound-containing ferrous phosphate and the manganese dioxide nanomaterial is mixed with a lithium salt and an adhesion promoter and reacted in an oxygen-free environment to obtain a lithium iron manganese phosphate precursor;

[0049] In the present invention, the "lithium salt" includes but is not limited to at least one of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, lithium hydroxide and lithium chloride. In general, the added molar amount of the lithium salt should satisfy the following requirements: + :Fe 2+ =(1.0-1.1):(0.2-0.8), suitably, Li + :Fe 2+ =(1.0-1.1):(0.2-0.3).

[0050] In the present invention, the "adhesion promoter" includes but is not limited to at least one of fatty alcohol polyoxyethylene ether sulfate, dicyclohexylamine, ethylene glycol, sodium lauryl sulfate and dimethylacetamide. Generally, the amount of the adhesion promoter added is 0.1% to 1% of the mass of the lithium salt, suitably 0.7% to 1%.

[0051] In the present invention, the oxygen-free environment can be formed by replacing the air therein with nitrogen and / or inert gas and continuously introducing nitrogen and / or inert gas.

[0052] S4. The lithium manganese iron phosphate precursor is sequentially granulated, dried, sintered and cooled to obtain the composite polycrystalline lithium manganese iron phosphate.

[0053] In the present invention, the particles obtained by the granulation can be spherical, cylindrical, etc., and the equipment used for granulation can be conventional equipment commonly used in the field and will not be described in detail here; in the present invention, the drying and sintering processes are both carried out in an oxygen-free environment, and the oxygen-free environment can be replaced by nitrogen and / or inert gas in which the air is continuously introduced; the sintering can be carried out in a tubular furnace, and the sintering temperature is generally 500°C to 850°C, suitably 600°C to 850°C, more suitably 700°C to 850°C, and the sintering time is adjusted accordingly according to the sintering temperature, generally 6 to 10 hours.

[0054] In the present invention, the cooling rate can be a conventional rate, such as 10°C / h to 50°C / h, until the temperature reaches room temperature. During the cooling process, the lithium manganese iron phosphate is in-situ grown on the carbon product to present a polycrystalline structure.

[0055] The present invention facilitates the structural change of aerogel into a nanoporous structure through sintering, providing good energy storage effect; the synergistic effect of aerogel and carbon-containing material helps stabilize the lithium ion channel, ensures the normal deintercalation of lithium ions, and greatly improves the cycle performance and discharge rate. At the same time, the lithium manganese iron phosphate prepared by the method disclosed in the present invention has controllability, consistency and good conductivity; the addition of a very small amount of adhesion promoter in the present invention can greatly improve the powdering phenomenon of the carbon layer during subsequent processing of the material, thereby increasing the conductivity and processing performance of the material. The lithium manganese iron phosphate prepared by the present invention has a uniform particle size distribution, excellent battery energy density and compaction density, and has good application prospects in the battery field.

[0056] Example

[0057] Example 1

[0058] 1) dissolving ferrous sulfate heptahydrate in deionized water to prepare a 2 mol / L ferrous sulfate solution, adding ascorbic acid at a weight ratio of 0.1% of the ferrous sulfate, adding two or more carbon-containing substances at a weight ratio of 10% of the ferrous sulfate, i.e., graphene: phytic acid = 6:4, mixing well, adding 105% of the theoretical amount of ammonium dihydrogen phosphate, and adjusting the pH value to 7 with concentrated ammonia water. Under nitrogen protection, the reaction system temperature is maintained at 50° C., stirring and reacting for 1 hour, and filtering with a vacuum pump to obtain a graphene-containing ferrous phosphate wet material.

[0059] 2) preparing a 2 mol / L potassium permanganate solution, heating the solution, adding a graphene aerogel containing 7% manganese by weight, and directly synthesizing a polycrystalline manganese dioxide nanomaterial by a hydrothermal method;

[0060] 3) adding the manganese dioxide nanomaterial obtained in step (2) to the graphene ferrous phosphate wet material obtained in step (1), and then adding Li + :Fe 2+ =1.1:0.6 of lithium phosphate, and ethylene glycol was added to the reaction system at the same time, and the mixture was fully stirred under nitrogen protection to obtain a lithium manganese iron phosphate precursor;

[0061] 4) spray drying the precursor obtained in step (3) to obtain rounded spherical particles;

[0062] 5) The material obtained in step (4) is placed in a tubular furnace under nitrogen protection and sintered at 790° C. for 6 hours, and then cooled to room temperature to obtain a composite polycrystalline graphene lithium manganese iron phosphate product formed by in-situ growth of lithium manganese iron phosphate on the graphene layered structure.

[0063] The product obtained in Example 1 was subjected to SEM electron microscope examination. The results are as follows Figure 1 As shown, there are Figure 1 It can be seen that the particle size of the product obtained by the present invention is evenly distributed, which avoids the bulging phenomenon during the subsequent preparation of battery coating, and the material processing performance is improved.

[0064] When the discharge platform voltage is 3.4-4.2V, the charge and discharge capacity of the lithium manganese iron phosphate prepared in this embodiment is 150mAh / g. At a rate of 0.5C, the energy retention rate is 40% after 50 cycles, and the cycle performance is relatively good. The compaction density is 2.6g / cm 3 , indicating that the lithium manganese iron phosphate prepared by the method disclosed in the present invention has good charge and discharge performance and compaction density, and is worthy of promotion in the battery field.

[0065] Example 2

[0066] 1) dissolving ferrous sulfate heptahydrate in deionized water to prepare a 2 mol / L ferrous sulfate solution, adding ascorbic acid at a weight ratio of 0.1% of the ferrous sulfate, adding two or more carbon-containing substances at a weight ratio of 3% of the ferrous sulfate, i.e., tartaric acid: sucrose = 7:3, mixing evenly, adding 110% of the theoretical amount of phosphoric acid, and adjusting the pH value to 8 with concentrated ammonia water. Under nitrogen protection, the reaction system temperature is maintained at 80° C., stirring and reacting for 1 hour, and filtering with a vacuum pump to obtain a graphene-containing ferrous phosphate wet material.

[0067] 2) preparing a 2 mol / L potassium manganate solution, adding 105% of the theoretical molar amount of hypochlorous acid, adding a V2O5 aerogel with a manganese content of 7%, and directly synthesizing a polycrystalline manganese dioxide nanomaterial by a hydrothermal method;

[0068] 3) adding the manganese dioxide nanomaterial obtained in step (2) to the graphene ferrous phosphate wet material obtained in step (1), and then adding Li + :Fe 2+ =1.1:0.8 lithium phosphate, and ethylene glycol is added to the reaction system at the same time, and fully stirred under nitrogen protection to obtain a lithium manganese iron phosphate precursor;

[0069] 4) spray drying the precursor obtained in step (3) to obtain rounded spherical particles;

[0070] 5) The dried lithium manganese iron phosphate precursor obtained in step (4) is placed in a tubular furnace under nitrogen protection and sintered at 790° C. for 3 h, and then cooled to room temperature to obtain a composite polycrystalline graphene lithium manganese iron phosphate product formed by in-situ growth of lithium manganese iron phosphate on the graphene layered structure.

[0071] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing composite polycrystalline lithium manganese iron phosphate, characterized in that: The preparation method comprises: A phosphorus source and a carbon-containing substance in an amount of 3% to 10% by mass of the ferrous sulfate are added to a ferrous sulfate solution, reacted in an oxygen-free environment at a pH of 5 to 10, and filtered to obtain a wet ferrous phosphate material containing a carbon compound; the carbon-containing substance is a mixture of two carbon-containing substances having different particle sizes, one of which is graphene and the other is one of glucose, phytic acid, citric acid, sucrose, and tartaric acid; An oxidant and an aerogel are added to a manganese sulfate solution to react, thereby obtaining a manganese dioxide nanomaterial containing an aerogel; the aerogel is a graphene aerogel; and the amount of the aerogel added is 1% to 7% of the mass of manganese in the manganese sulfate solution; The graphene-containing ferrous phosphate wet material and manganese dioxide nanomaterial are mixed and reacted with lithium salt and adhesion promoter in an oxygen-free environment to obtain a lithium manganese iron phosphate precursor; The lithium manganese iron phosphate precursor is subjected to ball milling, drying, sintering and cooling in sequence to obtain the composite polycrystalline lithium manganese iron phosphate.

2. The method for preparing composite polycrystalline lithium manganese iron phosphate according to claim 1, characterized in that: In the process of preparing the lithium iron manganese phosphate precursor, the molar ratio of lithium ions in the lithium salt to iron ions in the ferrous phosphate wet material is 1.0-1.1:0.2-0.

8.

3. The method for preparing composite polycrystalline lithium manganese iron phosphate according to claim 1, characterized in that: The concentration of the ferrous sulfate solution and the concentration of the manganese sulfate solution are both 1-5 mol / L.

4. The method for preparing composite polycrystalline lithium manganese iron phosphate according to claim 1, characterized in that: An antioxidant is also added in the step of preparing the graphene-containing ferrous phosphate wet material.

5. The method for preparing composite polycrystalline lithium manganese iron phosphate according to claim 1, characterized in that: The mass ratio of the two carbon-containing materials with different particle sizes is 3~7:1~6.

6. The method for preparing composite polycrystalline lithium manganese iron phosphate according to claim 5, characterized in that: The mass ratio of the two carbon-containing materials with different particle sizes is 3~7:4~6.

7. The method for preparing composite polycrystalline lithium manganese iron phosphate according to claim 1, characterized in that: In the step of preparing the graphene-containing ferrous phosphate wet material, the reaction temperature is 50-80° C.; the amount of the phosphorus source added is 100%-110% of the theoretical added mass; in the process of preparing the manganese dioxide nanomaterial, the amount of the oxidant added is 100%-110% of the theoretical molar amount; the adhesion promoter includes at least one of sodium fatty alcohol polyoxyethylene ether sulfate, dicyclohexylamine, ethylene glycol, sodium lauryl sulfate and dimethylacetamide; the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid; the lithium salt includes at least one of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, lithium hydroxide and lithium chloride; and the oxidant includes at least one of oxygen, air, ammonium persulfate, hypochlorite, chlorate and permanganate.

8. A composite polycrystalline structure of lithium manganese iron phosphate, characterized in that: The composite polycrystalline lithium manganese iron phosphate is prepared by the preparation method of any one of claims 1 to 7.

9. The composite polycrystalline lithium manganese iron phosphate according to claim 8, characterized in that: The general chemical formula of the lithium manganese iron phosphate is: LiMn x Fe y PO4 / C, where: 0.2≤x≤0.9, x+y=1.

10. Use of the composite polycrystalline lithium manganese iron phosphate prepared by the method for preparing the composite polycrystalline lithium manganese iron phosphate according to any one of claims 1 to 7 or the composite polycrystalline lithium manganese iron phosphate according to any one of claims 8 to 9 in a lithium battery.

Images